%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-27 on page II-145 in "Vertical space in and around displays".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.7\abovedisplayskip}%
    \setlength\belowdisplayskip{.7\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage{mathtools}
%StopShownPreambleCommands

\begin{document}
A \texttt{gather} with normal spacing:
\begin{gather}
  A=\sum_{i=1}^n x_i \\ B = x_1 + x_2 + x_3
\end{gather}
And now spread farther apart:
\begin{spreadlines}{15pt}
  \begin{gather}
    A=\sum_{i=1}^n x_i \\ B = x_1 + x_2 + x_3
  \end{gather}
\end{spreadlines}
\end{document}
